Apparatus for improving the reception of telegraph signals



L. RICHEY IMPROVING THE RECEPTION LEGRAPH SIGNALS Filed Sept. l9, 1945 July 4, 1950 .J. APPARATUS FOR OF TE I m w l 5 5 J 3 525mm 4 3 4 v 2 m )6 l. 2 2 a 5:2: 3 8 a 2 523mm Er=dz 523mm 513152 992 3 6 99E 25 m K 2 A m K 2 A MA JOSEPH L. RICHEY Mw w AW Patented July 4, 1950 I "APPARATUS FOR IMPROVING THE RECEP- TION OF TELEGRAPH SIGNALS Joseph L. Richey, United States Navy Application September 19, 1945, Serial No. 617,402

' 6 Claims. (c1. 250-8) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) This invention relates in general to telegraph circuits and more particularly to radio telegraph circuits in which the virtual signal to noise ratio has been increased, enabling the operator to receive signals that hewas unable to receive before the application of this invention.

In the older telegraph systems. the circuits were designed so that current flowed in the line during the mark signal interval and the current ceased to flow during the spacesignal interval. Actually, noise currents flow during the space signal interval due to electrical? disturbances in the transmission medium. In any given manual radio telegraph system the mark signal current flow must be strong enough so that the operator can distinguish the intelligence over the background noise. The ratio of mark to space current is generally referred tc as the signal to noise ratio. This'type of system is known as on-off keying.

In the more modern telegraph systems the signal to noise ratio has been improved by using frequency-shift keying. Frequency-shift keying consists in transmitting a continuous carrier wave of suitable frequency and changin this frequency between certain limits for the mark and space signals. In other words the carrier wave frequency is modulated by the telegraph signals. At the receiver, the frequency modulated carrier wave is reproduced and passed through a limiter to remove amplitude variations developed during transmission. The overall effect of the frequency shift keying is an increase in the virtual signal to noise ratio due to the removal of some of the disturbancebythe limiter, and polar. action.

This inventionis concerned with an additional increase in the virtual signal to noise ratio for the two systems described above. his not to be confused with other noise reducing apparatus such as the expander noise reducer'employing elements such as varistors. This type of reducer may be cascaded with that of this invention to produce a maximum overall effect. This invention consists in deriving a bias voltage to vary the gain of the controlled amplifier and to reduce conduction in the amplifier during the space interval. By having little or no conduction in the controlled amplifier during the space interval the background noise is reduced, which results in an overallincrease in the virtual signal to noise ratio, and enables the operator to receive border line signals that were very difficult. to receive before.

It is an object of this invention to produce a system for increasing the virtual signal to noise rati in a communication circuit.

Another object is to produce a system for increasing the virtual signal to noise ratio in a manually operated radio telegraph system.

Another object is to produce a system for increasing the virtual signal to noise ratio in onoff keying and frequency shifts keying radio telegraph circuits.

' Another object is to produce a noise reduction system which will be economical to'manufacture, reliable in operation and which possesses all of the qualities of ruggedness and dependability in serivec. I Figure 1 illustrates a circuit for increasing the virtual signal to noise ratio in a radio telegraph system using on-off keyed signals.

' Figure 2 illustrates a circuit for increasing the virtual signal to noise ratio in a radio telegraph system using frequency-shift keyed signals.

Referring to Figure 1, radio telegraph receiver 2 is connected to input antenna I and output leads 3 and 4. The output leads are connected to the grids of the controlled amplifier consisting of two triodes 'l and 8 in push-pull. Output transformers I0 and l I combine the outputs from the separate tubes in one circuit l2 to operate a loudspeaker or any similar indicating Battery 9 furnishes the plate voltage for the two tubes.

To develop an operating bias for the controlled amplifier, leads 2! and 122 divert a portion of the output of receiver 2 to bias amplifier 20 for amplification. The amplified output from 20 passes through band pass filter 23 to 26 and rectifier 21, where it appears as a D. C. voltage across resistor Hi, the direction of the current being shown by the arrow 42 whereby the switch contact l5 becomes sufiiciently positive to overcome the cutoff bias furnished by battery is thereby permitting the tubes 'l-8 to conduct. The D. C. voltage or bias voltageacross I 8 is fed to the grids 5 and 6 of the controlled amplifier through switch I ll5.

Battery 19 furnishes the bias to give the controlled amplifier a loss of amplification during the space signal when there is no voltage across resistor l8. To tune radio receiver 2 when this invention is utilized, switch arm I4 is moved to contact I6 which contact is furnished a bias by battery I! for tuning purposes. I

To operate the circuit of this invention, switch arm I4 is placed on contact IE to supply the tuning bias and the radio receiver is tuned to the desired transmitting station. The beat frequency oscillator is adjusted to give a maximum reading on ammeter 35, this increases a receiver tone device.

space interval has been reduced by the reduced conduction in the controlled amplifier by the bias from battery l9. creased virtual signal to noise ratio.

Referring to Figure 2, radio telegrapllreceiyer.

2 is connected to input antenna I and output leads 3 and 4. The output leads of'theirec'eiver:

1b The overall effect 'an 'in f 2 are connected to limiter 36 which functions to a give the frequency modulated carrier wave a constant amplitude. 1 is connected to the grids of the controlled am; plifier, consisting of twoitriodes l and 8 in pushpull. output transformers l and H combine the outputs from the separate tubes in one circuit 12 to operate a loudspeakeror any similar indicating, device. Battery. 9. furnishes the plate voltage for thetwo tubes.

, To-idevelop anoperating biasfor thecon-trolled amplifier, leads 2! and 22 divert a portion. of the output'of receiver 2 tobias, amplifier 29 for amplification. In :frequency shift; keying the frequenciesfor markand space- .signals, are gen erally approximately 850 cycles apart andthe signals are usually placed in the 2000. toBOOO cyclerange, Disoriminator 28 to 34 has its primary 2 8 -3; I-.-.-.3,2 tunedtotheimidband frequency of 2550 cycles, wherethe markirequency is 2125 cycles andthe Spacefrequency. is 2975 cycles. The secondary .of the discriminator has one section 22-3! tuned to-the space frequency of 2975 cycles and the other section 30-32 tuned to the mark frequency of 2125 cycles. For a space signal frequency current will .flow in. tuned circuit; ?.;93.I developing a voltage across condenser 29 which causes a direct current to flow through ammeter-35, resistance l8. and rectifier. 3.3, the direction of the current flow being shown by the arrow. The direct current develops a voltage acrossresistor 18 such that the switch contact I becomes more negative than before. This negative potential at l5 is-applied togrids5 and 6 of the-controlled amplifier, biasing tubes land 3 tocut-off, thus introducing a loss in thecircuit. This loss not only prevents the space signal frequency'of 2975 cycles from being audible in the loudspeaker, it, also prevents the'normal back.- ground noise of the space signal from being audible :inthe loudspeaker. Fora mark signal frequency current will flowin tuned circuit 30- 32, developing a voltage across condenser 36 which causes a direct current to flow through rectifier 34, resistance [8, and ammeter .35, as is shown by the arrow 4|. Thisdirect current develops a voltage across resistor I8 such that the switch contact becomes sufiiciently positive toovercome the negative .bias of the battery is. This positive potential at I5 is applied toqgrids of thecontrolled amplifier, biasing. tubes land '8 "forhigh gain, thus introducing atransmission gain in the circuit. This transmission gain causes the mark signal. frequency of 21-25 cycles to becomeaudible in the loudspeaker ofheadset, thus enabling the operator to receive the code i na Battery 19 in this circuit furnishes a bias to adjust the developed bias across resistor l8, to the desired point on the characteristic ,curve.

The output of thelimiter To tune radio receiver 2 when this invention is utilized, switch arm I4 is moved to contact l6 which is furnished a bias by battery I! for tuning purposes.

To operate the frequency shift circuit of this vent o switc sa Mis ace n, Contact to supply thetuning bias, and the radio receiver is tuned to'the desired transmitting station. The beat frequency oscillator is adjusted to give a maximum reading on ammeter 35 while in the mark condition; thisiinsures a receiver tone output' ofthe mark frequency. Switch arm [4 is now, moved to contact [5 and battery l9 adjusted until the developed bias across resistance I8 is 15'.

operating. on'sthatzpoin't of the characteristic curve Where, the background noise of the space signal is a-minimum. The circuit is now ready for operatiomatan increased virtual signal to noise ratio, because the background noise of the space signal has been materially reduced by the developed negative bias, acting on the controlled amplifier .to -,insert .a.,transr;nission loss in the ci r cuit dur ng thespace interval. The overall effect is, an increasedv virtual. signal to noise ratio.

The invention. described herein may be made or used by or for thelqovernmentgf the-Un t States for governmental purposes withgut the paymentto .m oi any revaltiesthereono there-r fors ,What isclaimedisz 1. Apparatus for receiving radio, telegraph sig-,. nals wherein the controlled amplifier functions i c s t avi ua s s-ml t liq ti co e prising,..,a..radio receiver, a controlled amplifier e tl re eivin ts nput r m th o ti t o the radio receiver, a biasing amplifier also receiving its input from theoutput of the'radi rec e iver,,a band pass filter. for selecting the mark signal current frequency from theoutput of the bias amplifier, arectifier circuit for developing a, grid voltage for the controlled amplifier to cause a transmission gain in the signal circuit fora mark signalanda battery for developinga grid voltage for the controlled amplifier to'ica'use reduced conduction in the signal circuit for a spaces'i gnal,

2. Apparatus for receiving radio telegraphsige nals wherein the. controlled amplifierLfunctionsto increase the virtual signal to noise ratio comprising, a radio receiver, a controlled amplifier directlyreceiving its input from the output of ther-adio. receiver, a. biasing amplifieralsoireceiv ing its input from theoutput of the radio. receiver,

a discriminator for segregating the mark and space signal ,currentsfrom the. outputofthe bias amplifier, arectifier circuit included in the \discriminator .for developinga relatively low :g'r id voltageimpressible onthe controlled amplifier circuit. to cause a transmission gain in said sis": nal'cirjcuit for a mark signal, and another rectifier ,circuit included inthe discriininatorfor vel in a rela lvihi h rid vo ta e al o impressible on the controlled amplifier circuit to cause reduced conduction in said signal cir-, ui ras a s snal ad o e eg am si na lin ap atus a c ver ha n condu to l a s a. n rpl s m-I pl fie 'ac u an indicating vise! lled am l r in l di ig le at te sr d of whi h the eads e min te sou ce bf i 'w r imr s ne tive bias 9n the'grids of said triodes thereby to cause re du d sndu ti n th v c ntrolled am ifie s ui todenp e space si na s. means t9 fig.- nish an tin bi s t r the .csnt sd plifier comprising a bias amplifier connected to said leads to divert a portion of the receiver output, and filter and rectifier means serially connected with the bias amplifier and unit source of direct current to dominate said unit current source and thereby cause increased conduction in the controlled amplifier to denote mark signals.

4. In radio telegraph signalling apparatus, a receiver having conductor leads, a controlled amplifier to actuate an indicating device, said controlled amplifier including triodes at the grids of which the leads terminate, a bias amplifier connected to said leads to derive an operating bias therefrom by diversion of a portion of the receiver output, a discriminator connected to the bias amplifier and tuned to pass mark and space signals of dissimilar frequencies, and rectifier circuits connected with the discriminator said circuits embodying rectifier means impressing positive direct current on said grids to bias the triodes into conduction for a mark signal, and negative direct current on said grids to bias the triodes to cut-01f for space signals.

5. Apparatus for receiving radio telegraph signals wherein a controlled amplifier functions to increase the virtual signal to noise ratio comprising, a receiver for successive mark and space radio telegraph signals, said receiver having conductor leads, a controlled amplifier consisting of a plurality of space discharge devices having at least an anode, cathode, and control grid, the said control grids being directly connected to the receiver conductor leads, a biasing amplifier also directly connected to the conductor leads, a bandpass filter connected to the output of the biasing amplifier for segregating the mark from the space signals, a rectifier means serially connected to the bandpass filter so as to produce a direct current indicative of the mark signals, and a unit source of direct current, also connected to said control grids for imposing a fixed negative bias on the controlled amplifier thereby 6 reducing conduction to denote space signals, the rectified direct current dominating said unit source of direct current to cause increased conduction to denote mark signals.

6. Apparatus for receiving radio telegraph signals wherein a controlled amplifier functions to increase the virtual signal to noise ratio comprising a receiving means for successive mark andspace telegraph signals, a controlled amplifler consisting of a plurality of space discharge devices having at least an anode, cathode, and control grid, the said receiving means being di rectly connected to the said control grids, a biasing amplifier also directly connected to the receiving means, a discriminator seriall connected to the bias amplifier and tuned to pas-s mark and space signals of dissimilar frequencies, rectifier circuits connected with the discriminator and directly to said control grids of the controlled amplifier, said circuits embodying rectifier means for alternately impressing positive and negative direct currents on said control grids whereby the conduction of the controlled amplifier is increased or reduced as indicative of the mark or space signals.

JOSEPH L. RICHEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,795,393 Herman Mar. 10, 19:31 2,283,404 Wood May 19, 1942 2,306,214 Hanley Dec. 22, 1942 2,356,390 Finch Aug. 22, 1944 2,362,958 Sandretto Nov. 14, 1944 2,370,216 Worcester, Jr. Feb. 27, 1945 2,373,495 Olsen Apr. 10, 1945 2,379,799 Haigis July 3, 1945 2,383,126 Hollingsworth Aug. 21, 1945 2,397,830 Bailey Apr. 2, 1946 

